I created a custom column called "Sequence" with Lookup name of "sequence".
I modified (quickly) my regex to extract "sequence" instead of "Series" and "series_index".
Does Calibre not allow "Add books" to load metadata into custom columns?
Also the "test functionality" doesn't allow you to test custom column metadata extraction?
If it is any help, I am using the latest portable Calibre?
My modified regex:-
((?P<sequence>.*)\s*,\s*)?\s*(?P<title>.+)\s* - \s*(?P<author>.+)
And the book filename:-
ABC, ttttiiiittttlllleeee - A N Author.txt
The "ABC" doesn't appear in the "Sequence" custom column.